Refractive corrections for human eyes can be characterized into two general categories. The first category is the conventional method of vision correction which corrects for the eye's focus error and cylindrical error as measured using a manifest refraction. The second category is wavefront-guide vision correction which provides correction for all aberrations in an eye, including focus error, cylindrical error, spherical aberration, coma, and others, measured using an objective wavefront sensor.
The conventional method of vision correction is conceptually limited to a correction of just focus error and cylindrical error. In addition, it is also constrained by the subjective nature of how the manifest refraction determines the eye's refractive errors, particularly the eye's cylindrical error. Cylindrical error is also known as astigmatism, and it causes particular problems because it includes both a cylindrical power and a cylindrical axis.
There are at least five limiting factors associated with a manifest refraction. First, manifest refraction is limited by available lenses in a phoroptor because a manifest refraction relies on applying corrective lenses and testing vision of the eye subjectively. Focus error is usually limited to a resolution of 0.125 Diopters (D) while the cylindrical error is limited to a resolution of 0.25 D. Second, subjective determination of cylindrical axis can be problematic because a slight variation of cylindrical axis—within only a few degrees—can cause a significant performance difference for a cylindrical correction of more than 2 D. Third, human errors by either the patient or a practitioner such as an optometrist or optician cannot be excluded because a manifest refraction involves the subjective responses of a patient to a plurality of refractive corrections, as well as the practitioner's analysis of those subjective responses. Fourth, a manifest refraction is fundamentally a partial empirical refractive solution, because a practitioner conducting the manifest refraction determines an end point for a refractive correction in a time-consuming process. Finally, manifest refraction can also be a time consuming process because it relies on human control of vision optimization with as many as three independent variables which include a focus error, a cylindrical power, and a cylindrical axis.
The drawbacks associated with using a manifest refraction compound with the high tolerance of current lens manufacturing techniques and lead to widespread erroneous vision correction. The inaccuracy of the conventional vision correction method using a manifest refraction leads to a situation where there may be significant differences in a refractive prescription of the same eye by different practitioners, as well as in a coarse resolution of cylindrical power—as large as 0.25 D—universally prescribed for conventional vision correction. Consequently, available ophthalmic lenses in today's ophthalmic industry are also limited to lenses in 0.25 D resolution. Correcting an eye's astigmatism using conventional vision correction is further complicated by the high tolerance in fabricating conventional spectacle lenses. Moreover, it is accepted in the industry that visual acuity of 20/20 is perfect already with no need for correction.